1. Field of the Invention
The present invention relates to a power steering system for assisting a steering power of a wheel and to a control device of an electric motor which attempts improvement of a steering feel at lock end of a steering wheel.
2. Related Background of the Invention
As a power assist device for assisting power of a wheel steering according to a steering torque input from a driver, a hydraulic mechanism is usually used. As an example of this prior art, such a technology is proposed that a hydraulic pressure is generated by driving an oil pump using an electric motor and the steering torque inputted from a steering wheel to this generated hydraulic pressure is feedback-controlled so as to generate a steering assist power (See Patent Document 1, for example). With the prior art as shown in Patent Document 1, a steering torque is detected from a value of a torque sensor and the steering assist power is generated according to the torque so as to control a pressure to be supplied to a hydraulic cylinder.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-212141
However, with the prior art shown in the above Patent Document 1, the electric motor is controlled in proportion to the detected steering torque all the time, no matter whether the steering state is in a normal steering state or a lock end state. Therefore, a correcting method for an assist power in a region where a characteristic on a load side is changed such as, for example, at the lock end of the steering wheel, or a control method of an electric motor which can improve a steering feel at the lock end of the steering wheel has not been given sufficient consideration.
Generally, when the steering wheel is brought to the lock end, if a vehicle is moving forward, a steering tire generates a torque to return the steering wheel to a neutral position direction (A return torque is generated by a reaction force from the tire, which will be described in FIG. 2). On the other hand, if the vehicle is going backward, the steering tire generates a torque to further turn the steering wheel in the turning direction. Therefore, if the steering wheel is returned from the lock end in the backward movement of the vehicle, a power steering system needs to generate the largest steering assist power against the torque from the steering tire.
However, with the prior art, a torque generated by distortion between the steering wheel and the load side is detected by a torque sensor, and a steering assist power in proportion to the detected torque is generated using only the detected torque information. In this case, if a turning speed by the driver to return the steering wheel exceeds a predetermined value, a flow rate of working oil flowing from an assist-side (high-pressure side) cylinder to a non-assist side (low pressure side) cylinder becomes excessive, the working oil flows excessively through a relief valve provided at a pump, and thus a fluid flow phenomenon such as surging and water hammer occurs in a transient manner, which causes a problem of noise.